Something about MongoDB really drives the Postgres community (and certain NoSQL DB fans) nuts and I'm guessing it's that MongoDB is eating their lunch, growing faster than them, is gaining popularity faster, etc. Keep in mind MongoDB is many years younger too and is maturing more quickly now. Plus, developers absolutely love working with it.
It's a tool like any other and still has issues but I'm afraid some here are dancing on its grave well before it has even shown signs of letting up on its growth.